In catalytic reforming processes a platinum, or polymetallic platinum catalyst constituted of an alumina base to which one or more metals have been added with platinum to promote, modify and improve catalyst performance are conventionally employed. Reforming is defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics; dehydrogenation of paraffins to yield olefins; dehydrocyclization of paraffins and olefins to yield aromatics; isomerization of n-paraffins; isomerization of alkylcycloparaffins to yield cyclohexanes; isomerization of substituted aromatics: and hydrocracking of paraffins which produces gas, and inevitably coke, the latter being deposited on the catalyst. This sequence of reactions occurs across one or a sequence of reactors; typically a sequence of three or four reactors, each of which contains a bed, or beds, of the reforming catalyst. Typically each of the reactors receives downflow naphtha feed, and each is provided with a preheater or interstage heater because the reactions which take place are endothermic.
The activity of the catalyst gradually declines due to the build-up of coke. Coke formation is believed to result from the deposition of coke precursors such as anthracene, coronene, ovalene, and other condensed ring aromatic molecules on the catalyst, these polymerizing to form coke. During operation. the temperature in each of the reactors is gradually raised to compensate for the activity loss caused by the coke deposition. Eventually, however, it is necessary to reactivate the catalyst. Consequently, in all processes of this type the catalyst must necessarily be periodically regenerated by burning the coke off the catalyst at controlled conditions.
Regeneration processes are basically of two types, semi-regenerative and cyclic. In a semi-regenerative process, during the on-oil portion of the operating cycle the entire unit is operated by gradually and progressively increasing the temperature to maintain the activity of the catalyst, caused by the coke deposition, until finally the entire unit is shut down for regeneration, and reactivation, of the catalyst. After regeneration, and reactivation of the catalyst the unit is put back on-oil. In a cyclic regeneration process, the reactors are individually isolated, or in effect swung out of line by various manifolding arrangements, motor operated valving and the like. The catalyst is regenerated to remove the coke deposits, and then reactivated while the other reactors of the series remain on stream. A "swing reactor" temporarily replaces a reactor which is removed from the series for regeneration and reactivation of the catalyst, until it is put back in series. The cyclic method of regeneration offers advantages over the semi-regenerative type process in that it can be continuously operated because the catalyst can be regenerated, and reactivated, without shutting down the unit. Hence, there is no loss in production, and the unit can be operated at higher severities to produce higher C.sub.5 + liquid volume yields of high octane gasoline than semi-regenerative reforming units.
The net result in either type of regeneration however is the same. The coke must be oxidatively burned from the catalyst at temperatures ranging from about 400.degree. C. to about 800.degree. C., and the higher the required temperature, inter alia, the greater the damage to catalyst. Typically for example, the high temperature agglomerates the metal, or metals components of the catalyst, and redispersion of the dispersed metals at these high temperatures is virtually always required. For example, platinum-iridium catalysts are the most active of commercial reforming catalysts. Thus, iridium has the ability to promote the platinum activity and provide catalytic activities two to four times that of the more conventional platinum and, e.g., the now widely used platinum-rhenium catalysts, respectively, depending upon the platinum and iridium loadings. Unfortunately however, platinum-iridium catalysts unlike the platinum and platinum-rheniun catalysts all too readily agglomerate and inactivate upon exposure to oxygen at high temperatures. For this reason the wide application of platinum-iridium catalysts in commercial operations has been restricted, especially to exclude their use in cyclic reforming units, since time-consuming and inefficient regeneration procedures are required to avoid damaging the iridium. Nonetheless, albeit the high temperatures, of regeneration that is required is less damaging to reforming catalysts other than platinum-iridium catalysts, e.g., platinum and platinum-rhenium catalysts, a lower temperature catalyst regeneration process is needed by the industry.
Attempts have been made to regenerate zeolite catalysts at low temperature, as disclosed by (1) Copperthwaite R. G. et al, J. Chem. Soc., Chem. Commun. 1985, p 644-645; (2) Copperthwaite R. G. et al. J. Chem. Soc., Faraday Trans. 1, 1986, 82, p 1007-1017; and (3) Hutchings G. J. et al, Applied Catalysis, 34, 1987, p 153-161. These attempts however do not appear to have been successful; or, at best, can be described as partially successful. In attempts to regenerate 1/16 inch LZY82 extrudates, as described by Hutchings G. J. et al, e.g., only the top portion of the treated bed of catalyst appears to have been regenerated at all, and then only partially regenerated. The bed appeared as a black core of coked catalyst surrounded by a white layer of partially regenerated catalyst. More carbon remained on the catalyst when it was regenerated with ozone, than remained on a similar catalyst regenerated with oxygen. In the regeneration of ZSM-5 powder, as desscribed in the two Copperthwaite R. G. et al publications, generally similar results were obtained. Insofar as known, there is no art suggesting, or describing the regeneration of a metal, or metals-containing catalyst. This, of course, may not be too surprising since it is known that metals catalyze the conversion of ozone to oxygen. The loss of the third oxygen atom reverts the ozone molecule back to oxygen. The regeneration of a coked catalyst with oxygen requires temperatures on the order of at least about 400.degree. C. to about 500.degree. C. or higher; a condition at which temperature alone will convert ozone to oxygen. It would be very advantageous, and indeed a need exists, for a low temperature process for the regeneration of coked platinum and polymetallic platinum catalysts.